The present invention relates to a composite substrate such as a power module and the like comprising mounted exothermic electronic parts such as semiconductor elements, particularly, to a ceramic composite substrate which has a structure comprising a ceramic substrate connected with a metal layer, and has excellent heat radiation property, mechanical strength and heat-cycle-resistance property.
Conventionally, ceramic composite substrates obtained by connecting a metal plate composed mainly of copper or aluminum as an electric conductive layer to the surface of a ceramic substrate made of Al2O3, AIN, BeO and the like having electric insulation property have been widely used as constituent parts of various electric appliances.
Of these conventional ceramic composite substrates, those using an Al2O3 substrate as a ceramic substrate cannot acquire excellent heat radiation property due to the low thermal conductivity of Al2O3, and those using a BeO substrate have high thermal conductivity and excellent heat radiation properties. The drawback, however, is that they are difficult to manage in production due to toxicity thereof. Composite substrates using an AIN substrate are excellent in heat radiation property because of the high thermal conductivity of AIN; however, their disadvantage is that they tend to crack by mechanical shock, and thermal load in repeated use under practical conditions due to the low mechanical strength of AIN.
On the other hand, ceramics containing mainly Si3N4 are materials which generally show excellent heat resistance even under the atmosphere of a high temperature of 1000xc2x0 C. or more, and have low thermal expansion coefficient and also excellent thermal shock resistance, in addition to an inherent high strength property. Consequently, the application of the ceramics as a high temperature structural material to various high temperature high strength parts has been tried.
Recently, studies have been done on a ceramic substrate to be used in a composite substrate, by utilizing the high strength property owned inherently by ceramics containing mainly Si3N4. For example, JP-B-No2698780, and JP-A No. 9-157054 disclose a trial in which insufficient thermal conductivity is compensated by enhancing the head radiation property of the whole circuit, in a composite circuit board comprising Si3N4 substrates connected with a metal circuit plate, by making the thickness of the Si3N4 substrate smaller than 1 mm.
However, it is believed that, even in a Si3N4 substrate having higher strength than that of AIN, cracking also tends to occur by mechanical shock in installation and mounting or by thermal shock by a heat cycle similarly to the AIN substrate, and practical use of the Si3N4 substrate is difficult when the substrate are excellent in heat radiation property because of the high thermal conductivity of AIN; however, their disadvantage is that they tend to crack by mechanical shock, and thermal load in repeated use under practical conditions due to the low mechanical strength of AIN.
On the other hand, ceramics containing mainly Si3N4, are materials which generally show excellent heat resistance even under the atmosphere of a high temperature of 1000xc2x0 C. or more, and have low thermal expansion coefficient and also excellent thermal shock resistance, in addition to an inherent high strength property. Consequently, the application of the ceramics as a high temperature structural material to various high temperature high strength parts has been tried.
Recently, studies have been done on a ceramic substrate to be used in a composite substrate, by utilizing the high strength property owned inherently by ceramics containing mainly Si3N4. For example, JP-B No. 269870 and JP-A No. 9-157054 disclose a trial in which insufficient thermal conductivity is compensated by enhancing the head radiation property of the whole circuit, in a composite circuit board comprising Si3N4 substrates connected with a metal circuit plate, by making the thickness of the Si3N4 substrate smaller than 1 mm.
However, it is believed that, even in a Si3N4 substrate having higher strength than that of AIN, cracking also tends to occur by mechanical shock in installation and mounting or by thermal shock by a heat cycle similarly to the AIN substrate, and practical use of the Si3N4 substrate is difficult when the thickness of the substrate is small. The reason for this is, for example, that in a process of fabricating a ceramic composite substrate into an apparatus, the composite substrate must be fixed to the main part of the apparatus by screwing and the like. However, the occurrence of cracking by a pressing force by the screw and by shock in handling is inevitable even in a Si3N4 substrate having excellent mechanical strength when the thickness thereof is small. When such cracking occurs, insulation failure occurs at the cracked part, and the composite substrate becomes unusable because of dielectric breakdown.
An object of the present invention is to provide a ceramic composite substrate which manifests no generation of cracking on the substrate even by mechanical shock or thermal shock, and has an excellent heat radiation property and heat-cycle-resistance property, in view of such conventional conditions.
The present inventors have studied and developed, for attaining the above-mentioned object, a Si3N4 substrate material having high thermal conductivity and high strength, and found that when the ratio of the thickness of the Si3N4 substrate to the thickness of the metal plate is set at a given value in a composite substrate obtained, fastening cracks and the like in a fabrication process can be dissolved, and heat-cycle-resistance property can be significantly improved, and that the heat radiation property of a composite substrate can be considerably improved by enhancing heat the conductivity of a Si3N4 substrate, leading to the completion of the invention.
Namely, a ceramic composite substrate provided by the present invention comprises a silicon nitride ceramic substrate having a thermal conductivity of 90 W/mxc2x7K or more and a three-point flexural strength of 700 MPa or more, and a metal layer connected to one major surface thereof, and in the composite substrate, the thickness tc of the silicon nitride ceramic substrate and the thickness tm of the metal layer satisfy the relation formula: 2 tmxe2x89xa6tcxe2x89xa620 tm.
Further, another silicon nitride composite substrate provided by the present invention comprises a silicon nitride ceramic substrate having a thermal conductivity of 90 W/mxc2x7K or more and a three-point flexural strength of 700 MPa or more, and metal layers connected to both major surfaces thereof, and in the composite substrate, the thickness tc of the silicon nitride ceramic substrate and the total thickness ttm of the metal layers on both major surfaces satisfy the relation formula: ttmxe2x89xa6tcxe2x89xa610 ttm.
In the above-mentioned silicon nitride composite substrate of the present invention, the silicon nitride ceramic substrate before connection of the metal plate preferably is warped such that the major surface on which semiconductor elements are mounted forms a concave surface, and specific warp degree thereof is preferably in the range from 10 to 300 xcexcm per 25.4 mm (inch) of the length of the substrate.
The silicon nitride ceramic substrate used in a silicon nitride composite substrate of the present invention contains a rare earth element in an amount of 0.6 to 10% by weight in terms of an oxide and at least one element selected from Mg, Ti, Ta, Li and Ca in an amount of 0.5 to 1.0% by weight in terms of an
Namely, a ceramic composite substrate provided by the present invention comprises a silicon nitride ceramic substrate having a thermal conductivity of 90 W/mxc2x7K or more and a three-point flexural strength of 700 MPa or more, and a metal layer connected to one major surface thereof, and in the composite substrate, the thickness tc of the silicon nitride ceramic substrate and the thickness tm of the metal layer satisfy the relation formula: 2tmxe2x89xa6tcxe2x89xa620 tm.
Further, another silicon nitride composite substrate provided by the present invention comprises a silicon nitride ceramic substrate having a thermal conductivity of 90 W/mxc2x7K or more and a three-point flexural strength of 700 MPa or more, and metal layers connected to both major surfaces thereof, and in the composite substrate, the thickness tc of the silicon nitride ceramic substrate and the total thickness ttm of the metal layers on both major surfaces satisfy the relation formula: ttmxe2x89xa6tcxe2x89xa610 ttm.
In the above-mentioned silicon nitride composite substrate of the present invention, the silicon nitride ceramic substrate before connection of the metal plate preferably is warped such that the major surface on which semiconductor elements are mounted forms a concave surface, and specific warp degree thereof is preferably in the range from 10 to 300 xcexcm per 25.4 mm (inch) of the length of the substrate.
The silicon nitride ceramic substrate used in a silicon nitride composite substrate of the present invention contains a rare earth element in an amount of 0.6 to 10% by weight in terms of an oxide and at least one element selected from Mg, Ti, Ta, Li and Ca in an amount of 0.5 to 1.0% by weight in terms of an oxide, and impurity oxygen in an amount of 2% by weight or less and Al in an amount of 0.2% by weight or less in terms of an oxide.
The Si3N4 substrate used in a ceramic composite substrate of the present invention will be described below. The ceramic substrate used in a composite substrate is required to be a compact sintered body simultaneously having high thermal conductivity and high strength property. The reason for lower thermal conductivity of a conventional Si3N4 sintered body is that impurities are solved in Si3N4 particles of the sintered body and phonons, and carriers for heat conduction, are scattered. Since Si3N4 is a sintering-resistant ceramic, the addition of a sintering aid which allows the formation of liquid phase at lower temperatures is necessary, and it is known that this sintering aid is solved in the particles to lower thermal conductivity.
Consequently, in the present invention, thermal conductivity of a Si3N4 sintered body is improved in addition to inherent excellent mechanical strength by selecting the kind of sintering aid and controlling the amount of the aid added in a given range, and the resulted sintered body is used as a ceramic substrate. Namely, in the present invention, a rare earth oxide and an oxide of at least one element selected from Mg, Ti, Ta, Li and Ca are used together as sintering aids for Si3N4.
A rare earth oxide is effective for higher thermal conductivity of a sintered body since the oxide is scarcely solved in Si3N4 particles. Of rare earth oxides, it is preferable to use oxides of Y, Yb and Sm, because they enable easy crystallization of the grain boundary phase. Crystallization of the grain boundary phase is effective for simultaneously attaining high strength and high thermal conductivity since strength at higher temperatures increases and scattering of phonons at the grain boundary phase is reduced by this crystallization. Further, the amount of these added rare earth oxides is preferably in the range from 0.6 to 10% by weight. When the amount is lower than 0.6% by weight, the liquid phase is not sufficiently formed and compaction does not progress in the sintering process. Consequently, porosity after the sintering increases and thermal conductivity decreases, and mechanical strength also lowers simultaneously. On the other hand, when the amount is less than 10% by weight, the proportion of the grain boundary phase occupying a sintered body increases and thermal conductivity decreases.
Other sintering aids, oxides of Mg, Ti, Ta, Li and Ca react with SiO2 on the surface of a Si3N4 particle at temperatures of 160xc2x0 C. or less to form a liquid phase, being effective to promote compaction in the sintering process. The addition of these oxides in a total amount of a low 0.5 to 1% by weight improves the sintering property significantly as compared with the single addition of rare earth oxides, and further, minimally affects the reduction in thermal conductivity. However, when the total added amount of these oxides of Mg, Ti, Ta, Li and Ca is over 1% by weight, there is the fear of a remarkable reduction in thermal conductivity by solving of these elements in Si3N4 particles. Further, when Mg, Ti, Ta, Li and Ca are contained in grain boundary phase components, an amorphous glass component is formed in the grain boundary phase, also leading to a reduction in thermal conductivity and a decrease in strength at higher temperatures.
Further, by enhancing the purity of a raw material powder, thermal conductivity of the resulting Si3N4 sintered body can be improved. It is widely known that oxygen and Al are easily solved in Si3N4 particles to reduce thermal conductivity. Therefore, in a Si3N4 substrate of the present invention, the thermal conductivity of a Si,N, sintered body can be further improved by controlling the amount of oxygen to 2% by weight or less and the amount of Al to 0.2% by weight or less, the oxygen and Al being used as impurities in a raw material powder of the sintered body, particularly in a Si3N4 powder.
The thus produced Si3N4 sintered body has excellent thermal conductivity properties at room temperatures of 90 W/mxc2x7K or more and a three-point flexural strength of 700 MPa or more. By increasing thermal conductivity beyond a conventional Si3N4 substrate having high thermal conductivity and simultaneously increasing the mechanical strength as described above, the thickness of a ceramic substrate can be increased to a level which can endure thermal shock and mechanical shock, while reducing the heat resistance of the whole ceramic composite substrate.
Namely, in a silicon nitride composite substrate of the present invention, a Si3N4 substrate having a thermal conductivity of 90 W/mxc2x7K or more and a three-point flexural strength of 700 MPa or more is used, and when a metal layer is connected to one major surface thereof, the thickness of the ceramic substrate is controlled such that the thickness tc of the Si3N4 substrate and the thickness tm of the metal layer satisfy the relation formula 1:2 tmxe2x89xa6tcxe2x89xa620 tm.
For improving the heat-cycle-resistance property of a ceramic composite substrate, it is effective to connect the metal layers on both major surfaces of the ceramic substrate, and in this case, the thickness of the ceramic substrate is so controlled that the above-mentioned thickness tc and the total thickness ttm of the metal layers connected on both major surfaces satisfy the relation formula 2: ttmxe2x89xa6tcxe2x89xa610 ttm. In this case, it is preferable that the above-mentioned relation formula 1 is also satisfied. The thicknesses of the metal layers to be connected on both major surfaces may be the same or different.
Regarding the above-mentioned relation of tc and tm or ttm, when the thickness tc of a Si3N, substrate is  less than 2 tm in the case of a metal layer connected to one major surface of the Si3N4 substrate, or when tc less than ttm in the case of metal layers connected to both major surfaces of the Si3N4 substrate, even a Si3N4 substrate having high strength as described above tends to crack by mechanical shock in mounting and manifests the generation of cracking by the heat-cycle. It is not preferable that tc is  greater than 20 tm in the case of a metal layer connected to one major surface, or tc is  greater than ttm in the case of metal layers connected to both major surfaces, since heat resistance of the whole composite substrate increases under this condition.
The specific thickness of a Si3N4 substrate is preferably 1 mm or more, for preventing the generation of cracking and breakage by mechanical shock. However, when the thickness of a Si3N4 substrate is too large, the heat radiation property and heat-cycle property of the whole substrate decrease, therefore, it is desirable that this thickness is approximately 6 mm or less.
It is preferable that a Si3N4 substrate of the present invention has a warp such that the major surface on which semiconductor elements are mounted forms a concave surface at the early stage prior to the adhering of the metal layer. Further, it is preferable that this warp degree is in a range from 10 to 300 xcexcm per 25.4 mm (inch) of the length of the major surface of the Si3N4 substrate.
If a metal plate is connected to the major surface in a concave form in a Si3N4 substrate having such warp and heat source elements such as a transistor chip and the like mounted thereon, when the amount of heat generation of elements increases to cause an increase in temperature of the whole circuit, tensile stress is applied to the major surface on which elements are mounted and compression stress is applied to the reverse major surface of the Si3N4 substrate, by this heat. Resultantly, the Si3N4 substrate initially warped to the element mounting side forming a concave surface is deformed toward such a direction that gives a parallel relation to the major part of the apparatus, and close adherence of the apparatus with the substrate is improved. Therefore, heat resistance of the whole apparatus can be further reduced.